In general, it is known that an iron loss of the electrical steel sheet drastically rises as an excitation frequency becomes higher. However, the drive frequency of a transformer or a reactor is actually increased to make the size of an iron core small and efficiency thereof high. Therefore, heat generation due to the iron loss of the electrical steel sheet frequently becomes problematic.
A method of increasing Si content to enhance an intrinsic resistance of steel is effective to reduce the iron loss of the steel sheet. However, when the Si content in steel exceeds 3.5 mass %, workability considerably degrades. Hence, it is difficult to produce the electrical steel sheet by a production method utilizing a conventional rolling process. Therefore, various methods are proposed to produce steel sheets with a high Si content. For example, JP-B-H05-049745 discloses a method wherein siliconizing is carried out by blowing a non-oxidizing gas containing SiCl4 onto a surface of a steel sheet at a temperature of 1023˜1200° C. to provide an electrical steel sheet having a high Si content. Also, JP-B-H06-057853 discloses a method wherein a steel sheet having a high Si content of 4.5˜7 mass % and being poor in the workability is continuously hot rolled under appropriate rolling conditions to obtain a hot rolled steel sheet having a good cold rolling property.
As a method of reducing the iron loss except for the increase of the Si content, it is effective to reduce the thickness of the sheet. When the steel sheet is produced by a rolling process of a high-Si steel as a raw material, there is a limit in the reduction of the sheet thickness. To this end, there has been developed and already commercialized a method wherein a low-Si steel is cold rolled to a given final thickness and, thereafter, siliconized in a SiCl4-containing atmosphere to increase Si content in steel. Since it is made possible to give a gradient to the Si concentration in the thickness direction, this method is disclosed to be effective in the reduction of the iron loss at a high excitation frequency (see Japanese Patent Nos. 3948113, 3948112 and 4073075).
When the electrical steel sheet is used as a core material for a reactor, the iron loss property is important as mentioned above, but a DC superimposition property also becomes very important. The term “DC superimposition property” means a characteristic of lowering inductance when an excitation current of the core is increased. It is characteristically preferable that a reducing margin of the inductance is small even when the current is increased.
In the core using the electrical steel sheet, a gap (air gap) is formed in the core to improve the DC superimposition property. That is, the DC superimposition property is adjusted by designing the core instead of changing the characteristics of the electrical steel sheet itself. However, it is recently demanded to further improve the DC superimposition property. Because, the improvement of the DC superimposition property can decrease the core body and causes a merit capable of decreasing the volume and the weight. Especially, the decrease of the weight in the core mounted on a hybrid car or the like leads in the improvement of fuel consumption as it is so that it is strongly desired to improve the DC superimposition property.
However, there is substantially no approach to improve the DC superimposition property of the electrical steel sheet itself until now. Hence, the improvement is actually dependent upon the design of the core as mentioned above.
It could therefore be helpful to provide electrical steel sheets capable of improving the DC superimposition property of the core excited at a high frequency.